New AMS campaigns take 30-45 days to test and hone before they return high ROI. Ad CPC is higher in the beginning, and then lowers over time as the ad sees more success. Be prepared to spend more during the first month for testing than what you plan to spend as your standard monthly marketing budget. Also note, AMS does not always spend the full daily budget, and sometimes can go over the budget if the previous days have been lower spend. AMS is a more advanced platform than Facebook and not recommended as a first advertising attempt for new authors.
Avg. reader acquisition cost $4.48